We　have　established　a　novel　reaction　process　using　Mg　vapour　and　B　powder　(a　gas-solid　reaction)　to　synthesize　MgB2　powder　with　a　low　oxygen　content.　For　the　first　time,　we　tried　to　eliminate　the　oxygen　in　Mg　powder　with　a　specially　designed　furnace　tube　that　can　keep　the　sample　under　an　inert　atmosphere　throughout　the　entire　synthesis　process.　The　duration　of　heat　treatment　has　a　conspicuous　effect　on　the　phase　formation　of　MgB2　which　differs　greatly　from　the　conventional　solid　state　reaction　method.　At　short　durations　such　as　1　h,　unreacted　B　powder　is　ubiquitous　in　the　sample.　Full　formation　of　MgB2　is　achieved　at　3　h.　At　longer　times　such　as　4　h,　MgB2　decomposes　to　form　MgB4.　It　is　deduced　that　the　main　reason　for　the　unstable　MgB2　phase　formation　in　the　gas-solid　reaction　is　the　Mg　vapour　present　during　the　entire　process.　A　feasible　chemical　reaction　model　of　the　novel　gas-solid　reaction　has　been　established.　This　work　provides　an　important　foundation　to　obtain　almost　oxygen-free　MgB2,　which　can　finally　resolve　the　mystery　of　the　effect　of　oxygen　on　superconductivity　in　this　system.